Comments

To zoom in the images, use the QT GUI (compile OpenCV with WITH_QT flag). It will give you better UI controls, including zoom in/out.

2-3. Trackbars and other OpenCV UI elements are just some quick and simple solution to control your algorithm and set some parameters interactively. You can simply implement any transformation of the values (like you said), just remember the values you should use.

If you want a more serious interface (for a software to be released), you should ceate a more serious interface (I suggest to use QT).